The invention relates to aircraft. The invention relates more particularly to aircraft having an area-ruled fuselage to reduce drag of the aircraft at high subsonic Mach numbers.
It is well known that as an aircraft approaches the high subsonic flight regime, there is a steep rise in aircraft aerodynamic drag. The onset of the rise in drag results from local regions of sonic or supersonic flow that occur on parts of the aircraft because of the contour of the aircraft surfaces; such regions of locally sonic or supersonic flow typically arise at flight Mach numbers of about 0.8 or so for many aircraft. As the Mach number is increased beyond this threshold, the drag begins to rise at a steep rate.
It is known that the onset of this drag rise can be delayed to a higher Mach number by careful design of the aircraft fuselage and wing. In particular, it is known that so-called area-ruling of the aircraft fuselage can be effective in delaying the onset of the transonic drag rise. In accordance with this technique, the fuselage in the vicinity of the fuselage-wing interface is contoured so as to locally reduce the fuselage cross-section to compensate for the cross-section of the wing. The objective in area-ruling of a fuselage generally is to avoid a steep gradient in the total cross-sectional area of the aircraft in the longitudinal direction. Thus, the fuselage preferably has a relatively larger cross-sectional area forward and aft of the wing than it has in the area of the wing. As an example, the familiar xe2x80x9ccoke-bottlexe2x80x9d shaped fuselage has been employed for military fighter aircraft, in which the fuselage is narrowed in the horizontal direction at the fuselage-wing interface.
Area-ruling of the fuselage of a passenger aircraft involves a number of design considerations, not the least of which is the desire to provide adequate space for the passengers so that they will not be cramped. Unfortunately, the desire to area-rule the fuselage in the vicinity of the wing is at odds not only with the need to maintain adequate passenger seating space but also with other design features in this part of the aircraft. For instance, traditionally the wing-fuselage intersection of a low-wing passenger transport aircraft includes a large fairing defining the lower aerodynamic surface of the fuselage in the area below the center portion of the wing that passes through the fuselage. The fairing is needed in order to accommodate stowed landing gear, to house air conditioning units, for structural and aerodynamic reasons, and to protect the center fuel tank in the wing in the event of a landing with the landing gear not deployed. The fairing increases the fuselage cross-section at precisely the longitudinal station where it would be desirable to reduce the fuselage cross-section, i.e., at the wing-fuselage intersection. Consequently, at high subsonic flight Mach numbers (e.g., M=0.85 or above), the fairing contributes substantially toward overall aircraft drag.
On such an aircraft, area-ruling of other regions of the fuselage at the longitudinal stations corresponding to the wing""s maximum cross-sectional area can be effective in lessening the deleterious impact of the fairing and the wing with respect to transonic drag. Area-ruling of the fuselage in the horizontal direction is not practical, however, because it leads to inefficiencies in the use of the space in the fuselage for passenger seating. Accordingly, it has been proposed to area-rule a passenger transport fuselage in the vertical direction. For instance, an aircraft is described in U.S. Pat. No. 5,992,797, assigned to the assignee of the present application and incorporated herein by reference, in which area-ruling of the upper portion of the fuselage above the wing is employed in order to achieve a reduction in aircraft drag at high subsonic Mach numbers. However, the resulting aircraft, although closer to an optimum cross-sectional area distribution than an equivalent aircraft without such area-ruling, is still far from such optimum area distribution. Accordingly, any measures that could be taken to get even closer to the optimum area distribution without sacrificing other important design considerations would obviously be desirable.
While this goal is easy to state, achieving it is difficult in practice because of the many countervailing design constraints. One very important constraint is the need to protect the center fuel tank of the aircraft in the event of a gear-up landing. In such a landing, the aircraft will essentially slide on its belly on the runway, thus bringing the center fuel tank into close proximity with the ground. There must be adequate structure between the ground and the tank to prevent the tank from rupturing. The fairing described above traditionally plays an important role in this regard. Thus, the problem becomes how to achieve a greater extent of area-ruling of the fuselage in the vicinity of the wing-fuselage intersection, in view of the traditionally required fairing and the need to maintain adequate passenger space.
The present invention addresses the above needs and achieves other advantages by providing an airframe structure in which area-ruling of the fuselage is accomplished at least partially by dishing or sculpting the fuselage in the keel area below the center portion of the wing that passes through the fuselage. The traditional fairing can be eliminated or can be reconfigured so that it does not detract from the objective of area-ruling. In order to provide the protection for the center fuel tank normally provided by the fairing, the invention in one embodiment employs portions of the fuselage keel located forward and aft of the dished or sculpted portion; contact between the fuselage in this region and the ground is prevented by the forward and aft portions of the fuselage. These forward and aft portions can be formed as xe2x80x9cbumpsxe2x80x9d at the keel area of the fuselage. Advantageously, these keel bumps are located forward and aft of the longitudinal station at which the wing""s maximum cross-sectional area occurs, and hence do not hinder the objective of providing area-ruling at that station. In another embodiment, the center fuel tank is protected by a keel beam running beneath the lower surface of the fuselage in the dished or sculpted region; the keel beam thus hangs out in the free stream air in this region. In this embodiment, the keel beam and keel bumps can both be used to protect the center fuel tank.
In accordance with the invention, any air conditioning or environmental control system that may be used on the aircraft can be located outside of the space below the center portion of the wing. For example, the environmental control system can be located in a lower portion of the fuselage forward of the wing. In another embodiment directed toward a double-decker aircraft, the aircraft has a main passenger seating deck and forward and aft upper passenger seating decks located above the main seating deck and separated by a middle section of the fuselage having no upper seating deck. In this embodiment, the environmental control system can be located in the middle section above the main passenger cabin between the forward and aft upper decks.